A diesel engine which uses clean-burning fuels such as DME (dimethyl ether) and high cetane number LP gas instead of light oil is now attracting attention as air pollution abatement measures for diesel engines. These fuels are liquefied gas fuels unlike light oil which is a conventional fuel. Namely, liquefied gas fuels have boiling points lower than that of light oil, and have the nature of vaporizing at room temperature, whereas light oil exists as a liquid under atmospheric pressure at room temperature. For this reason, in the case of a diesel engine using liquefied gas fuel, after the stop of the diesel engine, liquefied gas fuel remaining in an injection system which is a fuel supply line section leading to a fuel injection nozzle leaks from a nozzle seat part of the fuel injection nozzle into a cylinder of the diesel engine and vaporizes in the cylinder. As a result, vaporized DME fuel is charged in the cylinder, and when the diesel engine is to be started at the next time, there is a risk that abnormal combustion such as knocking occurs and the diesel engine cannot be normally started and large vibrations and noise occur.
As one example of related arts for solving this problem, there is, for example, an injection system (for example, Japanese Patent No. 3111254) which includes at least one heating device and means for forming connection which permits a flow between at least one pressure guide section of an injection system and a fuel tank, after the stop of a fuel supply device (such as an injection pump). The injection system forms connection between at least one pressure guide section of the injection system and the fuel tank after the stop of the fuel supply device, heats at least a part of the pressure guide section to transfer remaining liquefied gas fuel such as DME to a vapor phase and produce a gas blow, and press the remaining liquefied gas fuel of the pressure guide section to the fuel tank by the gas blow. In addition, there is a diesel engine fuel system (for example, JP-A-11-107871) which detects three pressure parameters, atmospheric pressure, fuel tank internal pressure and fuel return pipe internal pressure, and retrieves remaining liquefied gas fuel to a fuel tank by using the difference between these pressures.
In each of the above-mentioned related arts, as means for retrieving the liquefied gas fuel remaining in the pressure guide section such as a fuel return pipe to the fuel tank after the stop of the fuel supply device, it is necessary to incorporate not only means for switching pipe passages by solenoid valves or the like, but also at least one heating device (Japanese Patent No. 3111254) or a pressure sensor for detecting the fuel return pipe internal pressure (JP-A-11-107871). Accordingly, in a fuel supply system for a diesel engine, there is a risk that means for retrieving remaining fuel after the stop of the fuel supply device becomes expensive and the main cause of increasing the cost of the fuel supply system.
The present applicant has previously proposed a liquefied gas fuel supply device including means for retrieving remaining liquefied gas fuel to a fuel tank by aspiration means using an aspirator, as means for retrieving liquefied gas fuel remaining in an injection system to the fuel tank after the stop of the fuel supply device (Application Number 2002-60829). This means for retrieving remaining fuel through the aspirator forms a ring-shaped flow of liquefied gas fuel including the aspirator by using as a driving source a feed pump for delivering liquefied gas fuel from the fuel tank in its original use, and sucks the liquefied gas fuel remaining in the injection system by suction force produced in the aspirator by the flow of the liquefied gas fuel, thereby retrieving the remaining liquefied gas fuel to the fuel tank. Namely, the liquefied gas fuel remaining in the injection system can be retrieved only by the means for forming the ring-shaped flow passage of liquefied gas fuel by the aspirator and the feed pump, whereby the liquefied gas fuel remaining in the injection system after the stop of the fuel supply device without the need to provide a heating device, a pressure sensor or the like. Accordingly, it is possible to construct the remaining-fuel retrieving means at low cost.
However, the present inventor has promoted further intensive research and discovered that if the liquefied gas fuel remaining in the injection system of the liquefied gas fuel supply device after the stop of the diesel engine is to be sucked into the fuel tank by the suction means using the aspirator, a comparatively long time is required to suck all the liquefied gas fuel remaining in the injection system. This is because the suction force by the aspirator having no driving source is weak compared to pumps or the like having driving sources. As a result, a certain extent of time is required to retrieve all the liquefied gas fuel remaining in the injection system of the liquefied gas fuel supply device, and if the diesel engine is again started after the diesel engine is stopped for a short time, the diesel engine is restarted in spite of the fact that all the liquefied gas fuel remaining in the injection system of the liquefied gas fuel supply device is not yet retrieved, so that there is a risk that abnormal combustion such as knocking occurs.
In addition, when the diesel engine is again started after the diesel engine has been stopped, liquefied gas fuel needs to be charged into the injection system of the liquefied gas fuel supply device before the diesel engine is started. Therefore, the diesel engine cannot be started while liquefied gas fuel is being charged into the injection system of the liquefied gas fuel supply device. In addition, since a long time is required to charge liquefied gas fuel into the injection system of the liquefied gas fuel supply device, there is the problem that the diesel engine cannot be rapidly started when the diesel engine and the liquefied gas fuel supply device are stopped.